Scanning system for ion implantation accelerators

ABSTRACT

In the scanning system disclosed herein, each of the deflection electrodes of an ion implantation accelerator is connected to a rotating sector-shaped electrode which, as it rotates, passes by and forms a capacitor with each of a series of charged stator electrodes, the stator electrodes being oppositely charged in alternation. Accordingly, an alternating potential of substantially triangular waveform is developed across the deflection electrodes, causing a linear beam scan and thereby providing a substantially uniform distribution of ions over the surface of a semiconductor body being implanted.

United States Patent Harrison [54] SCANNING SYSTEM FOR ION IMPLANTATION ACCELERATORS [72] Inventor: Stanley Harrison, Bedford, Mass. [73] Assignee: KEV Electronics Corporation, Wilmington, Mass.

[22] Filed: Nov. 10, 1970 [21] Appl. No.: 88,406

[52] US. Cl. ..328/229, 250/49.5 R, 328/233 [51] Int. Cl. ..H01j 29/70 [58] Field of Search...328/229, 233, 256; 250/495 R [56] References Cited UNITED STATES PATENTS 2,694,160 11/1954 Rea ..328/229 X 3,567,924 3/1971 Dijon et a1. ..328/233 X Schlesinger ..328/229 X Aug. 29, 1972 2,520,447 8/ 1950 Wideroe ..328/233 Primary Examiner-Roy Lake I Assistant Examiner-Palmer C. Demeo Attorney-Kenway, Jenney & l-lildreth ABSTRACT In the scanning system disclosed herein, each of the deflection electrodes of an ion implantation accelerator is connected to a rotating sector-shaped electrode which, as it rotates, passes by and forms a capacitor with each of a series of charged stator electrodes, the

stator electrodes being oppositely charged in alterna- 0 tion. Accordingly, an alternating potential of substantially triangular waveform is developed across the deflection electrodes, causing a linear beam scan and thereby providing a substantially uniform distribution of ions over the surface of a semiconductor body being implanted.

1 Claim, 4 Drawing Figures 2 Sheets-Sheet 1 INVENTOR STANLEY HARRISON 2 .ww $1M. m

FIG. I

H V SUPPLY Patented Aug. 29., 1972 SCANNING SYSTEM FOR ION IMPLANTATION ACCELERATORS BACKGROUND OF THE INVENTION This invention relates to ion implantation accelerators and more particularly, to such an accelerator in which the ion beam is scanned uniformly to obtain a substantially uniform ion flux.

l-Ieretofore, scanning of the ion beam in ion implantation accelerators has been accomplished electronically. Typically, some sort of integrating or feedback type circuit has been employed to generate a scanning voltage of generally triangular waveform which has then been applied to the beam deflection electrodes of the accelerator to obtain an essentially uniform ion flux over the surface of a semiconductor body being implanted. Such systems, however, are typically rather complex and, due to the high voltages involved, are somewhat subject to failure.

Among the several objects of the present invention may be noted the provision of a beam scanning system for an ion implantation accelerator which provides a substantially uniform ion flux over a significant area; the provision of such a system which does not require complicated electronics; the provision of such a system which generates a waveform having substantial linear portions; the provision of such a system providing precisely predetermined waveforms and voltage levels; the provision of such a system which is highly reliable and which is relatively simple and inexpensive in construction. Other objects and features will be in part apparent and in part pointed out hereinafter.

SUMMARY OF THE INVENTION Briefly, the scanner of the present invention is operative to deflect the beam of ions emerging from an ion implantation accelerator to obtain a substantially uniform distribution of ions over the surface of a semiconductor body being implanted. For this purpose, the accelerator employs at least one pair of deflection electrodes, spaced apart along an axis which is transverse to the accelerator axis. The scanning system employs a rotor having at least one sector-like plate together with a stator having at least two, substantially co-planar, sector electrodes. The stator electrodes are maintained at respective d.c. potentials which differ by a voltage sufflcient to significantly deflect the ion beam if applied across the deflection electrodes. The rotor is then rotated around an axis perpendicular to the coplanar stator electrodes with the rotor sector plate moving parallel and relatively closely adjacent to the stator electrodes so as to pass over and form a capacitor with each of the stator electrodes alternately. One of the deflection electrodes is electrically connected to the rotor plate and the potential of the other is predetermined in appropriate manner. The rotation of the rotor causes an alternating potential gradient to be generated between the deflection electrodes and this potential scans the beam so as to provide a substantially uniform distribution of ions along the respective transverse accelerator axis.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a somewhat diagrammatic illustration of an ion implantation accelerator employing a scanning system in accordance with the present invention.

FIG. 2 is a top view, with parts broken away, of one of a pair of rotary capacitor assemblies employed in the FIG. 1 apparatus.

FIG. 3 is a face view of a rotor element employed in the capacitor assembly of FIG. 2.

FIG. 4 is a face view of a stator element employed in the capacitor assembly of FIG. 2.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, there is indicated at 11 generally an ion accelerator for implanting ions intothe surface of a semiconductor body in an evacuated en vironment, such a semiconductor body or wafer being indicated at 13. As is known to those skilled in the art, the high energy level implantation of ions into the surface of a semiconductor material, such as silicon, is a very precise way of introducing dopant or impurity materials into the crystal lattice for the manufacture of semiconductor electronic devices such as transistors and diodes. As is also understood, it is highly desirable to be able to apply the ions relatively uniformly over the face of the semiconductor body, i.e., to provide a substantially uniform ion flux. For this purpose, the beam of ions emerging from the implanting accelerator is typically scanned in a raster so as to obtain a substantially uniform average ion distribution over a reasonable exposure time.

In the apparatus illustrated in FIG. 1, an ion beam 15, emerging from the analyzing or separating system 17 of the accelerator, is shown as passing between a pair of vertical deflection electrodes 21 and 23 and a pair of horizontal deflection electrodes 25 and 27. The conventional vacuum system enclosing these elements is not shown in order to simplify the drawings for the purpose of explanation. As is conventional, each pair of deflection electrodes is spaced apart along a respective axis which is transverse to the accelerator axis. In the following description, only a scanning system for applying a substantially triangularly shaped voltage waveform to the vertical deflection electrodes 21 and 23 is described. As will be understood, however, a similar system is provided for driving the plates 25 and 27, although this similar system is not described in detail. As is also understood, the two transverse scanning systems will be operated at different frequencies so as to prevent the possibility of the two synchronizing and developing relatively coarse Lissajous figures which would interfere with the obtaining of an essentially uniform average ion distribution.

In the apparatus illustrated, a substantially triangular voltage waveform is generated between the plates 21 and 23 by means of a pair of rotary capacitor assemblies 31 and 33 operating in conjunction with a high voltage d.c. supply 35. As is described in greater detail hereinafter, each of the capacitor assemblies 31 comprises a plurality of stator elements 51, to which charging potentials are applied by the supply 35, and a plurality of rotor elements 57 which are carried on and electrically interconnected by a respective conductive rotor shaft, 37 or 39. As may be seen in FIG. 1, each of the vertical deflection plates 21 and 23 is connected to the rotor of a respective capacitor assembly by means of a respective slip-ring 41 and 43 operating on the respective rotor shaft 37 and 39. The rotors are driven at a suitable speed by a motor 45, an insulating coupling 47 being interposed between the motor and the capacitor assembly 33 and an insulating coupling 49 being interposed between the two rotor shafts.

The stator elements 51 are substantially square as illustrated in FIG. 4 and, within each rotary capacitor assembly, are assembled in spaced parallel relationship by means of conductive spacers 53 and through bolts 54 (FIG. 2). As is explained hereinafter, the spacers 53 provide electrical interconnection between the several stator elements in each capacitor assembly as well as maintaining the desired separation.

As noted previously, the rotor in each capacitor assembly comprises a plurality of elements 57. These elements are disk-like, as may be seen in FIG. 3, and are mounted on the respective rotor shafts 37 and 39 separated by conductive spacers 40 and held by bolts 42 so as to be interleaved with the stator elements 51. Suitable insulated bearings (not shown) are provided for journaling the rotors within the respective stators. The bearings and the various spacer dimensions are selected to maintain substantially uniform spacings on either side of each rotor element 57 as illustrated in FIG. 2.

Both the circular rotor elements 57 and the square stator elements 52 are conveniently fabricated of conventional double-sided printed circuit board material, e.g., the so-called G-l epoxy fiberglass substrate with a layer of copper or alloy foil on both sides. In forming the rotor and stator elements, the copper foil is etched away to provide a respective electrode pattern. The pattern for the rotor elements is illustrated in FIG. 3 and the pattern for the stator elements is illustrated in FIG. 4. In both cases the same pattern is used on both sides of the circuit board and the patterns on the two sides are maintained in registration. In the drawings, the foil remaining is indicated by the shaded portions.

Referring now to FIG. 3, the foil pattern left on each of the rotor elements 57 comprises an annular hub 61 which electrically connects six generally sector-shaped plate or electrode portions 63. The portions of the respective shafts 37 and 39 passing through the rotor elements 57, is preferably substantially square in crosssection so as to prevent relative rotation of the rotor elements and each of the elements 57 is coirrespondingly apertured, as indicated at 65. Each of the discs 57 also includes a pair of bolt holes 67, for the bolts 42.

Referring now to FIG. 4, the pattern of foil left on each of the square stator elements 51 forms an inner annular hub 71 which electrically connects six generally sector-shaped electrodes 73 radiating therefrom. The electrode in the lower right hand corner, as shown in FIG. 3, includes a tab 75 providing a means for interconnection between the several elements 51 through the intermediate conductive spacers 53 and from thence to one of the terminals of the high voltage supply 35 as illustrated in FIG. 1.

Interleaved with the electrodes 73 are a similar number of generally sector-shaped electrodes 77. Electrodes 77 are electrically interconnected by means of an outer circumferential band as indicated at 79. This interconnected set of electrodes is also provided with a tab 81 which extends into a second corner of the square element 51 for permitting connection, through the respective spacers 53, to one of the terminals the high voltage supply 35. The center of each stator element is apertured, as indicated at 78, to provide clearance for the rotor shaft with its spacers.

As may be seen in FIG. 1, the interconnections between the various stator elements and the high voltage d.c. supply 35 are such that, in the assembly 31, the sector-shaped stator electrodes 77 are positively charged while the electrodes 73 which alternate therewith are negatively charged. In the assembly 33, on the other hand, a converse situation exists, that is, the stator electrodes 77 are negatively charged while the interleaved electrodes 73 are positively charged.

When the high voltage supply 35 is activated and the motor 45 is energized to rotate the two rotor assemblies, it may be seen that each rotor electrode 63 will pass by and form a capacitor with the stator electrodes 73 and 77 alternately. As will be understood by those skilled in the art, the electrostatic forces involved will cause an alternating or pulsating potential to be impressed upon the rotor blades since the stator electrodes 73 and 77 are oppositely charged by the supply 35. While air separation of the rotor and stator elements may be suitable for some application, the high voltages frequently encountered in ion implantation situations, make it preferable that the rotary capacitor elements be run while immersed in an oil dielectric.

Since the rotor electrodes 73 are approximately sector-shaped, as are the various stator electrodes, it can be seen that the increases and decreases in effective capacitor area will be approximately linear functions of time, once the various electrode overlaps have been established. It then follows that a substantially triangular waveform is generated. There will, of course, be some rounding at the corners of the waveform since there must necessarily be some finite separation between the rotor elements and the stator elements and thus, the overlap and initiation of capacitance growth and decay will not occur instantaneously. However, a close approximation of a triangular waveform is obtained, that is, a waveform having essentially linear portions of substantial duration.

For each particular design, some improvement in the degree and/or duration of linearity may be obtained by empirically varying the shape of the rotor electrodes 73 to improve the waveform. In particular, it may be noted that a slight dishing or concavity at the end of each of the rotor electrodes, as indicated at 81 in FIG. 3 will typically provide an improvement in the waveform by increasing the relative rate of change of effective capacitor area at the beginning and end of each linear portion of the triangular waveform.

As noted previously, each of the vertical deflection electrodes 21 and 23 is connected to a respective one of the rotor assemblies through the respective conducting shaft 37 or 39. Since the alternating stator electrodes 73 and 77 in the capacitor assembly 33 are charged in complementary fashion to those in the assembly 31, it will be understood that the waveforms generated in the respective rotor electrodes will be out of phase, it being assumed that all of the rotor electrodes 63 are in alignment. Accordingly, the deflection electrodes 21 and 23 will be driven by out-of-phase alternating or pulsating potentials having triangular waveforms, which is the preferred mode for a balanced scan. However, it should be understood that scanning may also be provided by applying a triangular waveform to only one of the scanning electrodes, the potential of the other being fixed or predetermined at an appropriate d.c. level.

While the arrangement illustrated is preferred, i.e., in which the output signal is taken from the rotor and the input voltages are applied to the stator, it should also be understood that the apparatus can also be operated with the opposite arrangement and a triangular waveform will still be generated. Likewise, while flat circular electrode arrangements are favored, an equivalent cylindrical configuration could also be used. In such a case the capacitor electrode plates would, of course, be cylindrical surfaces rather than flat.

While the capacitive loading presented by the deflection electrodes 21 and 23 will necessarily somewhat reduce the voltages generated in the capacitor assemblies 31 and 33, this loading does not tend to affect the shape of the waveforms generated since the waveform generating mechanism is itself capacitive in nature. Thus, the loading effect is essentially that of a capacitive voltage divider and the linearity of the various triangular waveform sections is preserved. An advantage of the capacitive waveform generating apparatus of the present invention is that the amplitude of the output waveform can be readily adjusted or programmed by varying the input voltage since a highly linear relationship exists.

As noted previously, the horizontal deflection electrodes 25 and 27 will typically be driven by a triangular waveform voltage generated in an essentially identical rotary capacitor system operating in a different speed range so that a two dimensional raster is obtained and the surface of the semiconductor body 13 being irradiated will receive a substantially uniform average ion flux.

A further advantage of using the present mechanically driven system for generating both sets of triangular waveforms is that there is essentially no tendency for the two output frequencies to synchronize or lock onto one another as is the tendency with wholly electronic circuits. There is thus no tendency to generate relatively coarse, stationary Lissajous figures which would produce a nonuniform ion flux at the target.

In view of the foregoing, it may be seen that several objects of the present invention are achieved and other advantageous results have been attained.

As various changes could be made in the above construction without departing from the scope of the invention, it should be understood that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is: g

1. In an ion accelerator for implanting ions into the surface of a semiconductor body in an evacuated environment, a scanner for deflecting the beams of ions emerging from the accelerator to obtain a substantially uniform distribution of ions over the surface of said semiconductor body, said scanner comprising:

a pair of deflection electrodes spaced apart along an axis transverse to the accelerator axis and also in the ev cuated environment;

a pair 0 rotary capacitor assemblies each of which includes a plurality of flat, circular rotor elements, each rotor element having a plurality of substantially sector-shaped capacitor electrodes, and a plurality of flat stator elements interleaved with said rotor elements, each stator element having a first plurality of substantially sector-shaped capacitor electrodes and a second plurality of substantially sectorshaped capacitor electrodes, said first plurality being circumferentially interleaved with said second plurality;

means for interconnecting each of said deflection electrodes with the capacitor electrodes of the rotor elements in a respective one of said rotary capacitor assemblies;

means for applying a constant, predetermined d.c. potential between said first and second pluralities of electrodes of the stator elements; and

means for rotating said rotor elements with respect to said stator elements, the sector-shaped rotor electrodes of one of said capacitor assemblies being phased with respect to the sector-shaped electrodes of the other of said capacitor assemblies to apply out-of-phase triangular waveforms to said deflection electrodes. 

1. In an ion accelerator for implanting ions into the surface of a semiconductor body in an evacuated environment, a scanner for deflecting the beams of ions emerging from the accelerator to obtain a substantially uniform distribution of ions over the surface of said semiconductor body, said scanner comprising: a pair of deflection electrodes spaced apart along an axis transverse to the accelerator axis and also in the evacuated environment; a pair of rotary capacitor assemblies each of which includes a plurality of flat, circular rotor elements, each rotor element having a plurality of substantially sector-shaped capacitor electrodes, and a plurality of flat stator elements interleaved with said rotor elements, each stator element having a first plurality of substantially sector-shaped capacitor electrodes and a second plurality of substantially sector-shaped capacitor electrodes, said first plurality being circumferentially interleaved with said second plurality; means for interconnecting each of said deflection electrodes with the capacitor electrodes of the rotor elements in a respective one of said rotary capacitor assemblies; means for applying a constant, predetermined d.c. potential between said first and second pluralities of electrodes of the stator elements; and means for rotating said rotor elements with respect to said stator elements, the sector-shaped rotor electrodes of one of said capacitor assemblies being phased with respect to the sector-shaped electrodes of the other of said capacitor assemblies to apply out-of-phase triangular waveforms to said deflection electrodes. 